Dispersion of powders into various solvents is an important discipline for numerous industrial applications ranging from paints to cosmetics. It is well known in nanotechnology that very small nano-materials have properties which significantly differ from their bulk counterpart. For nano particles a large fraction of the particles consist of surface atoms which lead to very high surface area per mass. Suspensions of powders of nano-crystalline materials in solvents, wherein the size of the particle agglomerates in the suspension are in the nano-regime are attracting a lot of attention due to the increased functionality provided. A number of factors determine the functionality of the materials including chemical composition, crystal form and crystallinity, size of crystallites and size of particles in suspension. Examples of size dependent properties are optical transparency, hardness, thermal and electrical conductivity, chemical reactivity and catalytic and electronic properties.
Various techniques have been developed for dispersing powders into different solvents including high shear rate mixing, sonication, milling and/or grinding techniques. One often encountered problem is that such techniques lead to colloidal suspensions, which are hazy and/or milky due to a relatively large particle size in suspension. When applied e.g. as a coating to the surface of a substrate, the haze of the coating fluid is maintained in the coating, which is undesirable in many applications. Further the relatively large size of the particle agglomerates or clusters in such coatings and/or colloidal suspensions may result in a reduced or non-optimal functionality.
Some descriptions of fine grinding by the use of small beads to obtain submicron dispersions are present in e.g. [Dobbs (US 2006/0003013 A1) and Hsu et al (US 2004/0251329 A1)]. Fine grinding is normally carried out in energy intensive grinding mills such as planetary mill, attrition mill, oscillating mill, ball mill, bead mill, and jet mill. These mills deliver huge amount of energy for particle breakage to produce particles below 10 μm. Besides size reduction these energy intensive grinding mills mill also induce structural changes near surface region where the solids come into contact under mechanical forces besides size reduction. The structural changes induces changes in crystallinity, crystallite size and lattice strain. [THILAGAN PALANIANDY 2008].
Although it is possible by fine grinding to obtain submicron dispersion certain application needs crystalline particles. For example the use of TiO2 in photocatalytic applications the activity is directly related to the crystallinity of the TiO2 particles [Jensen et al. 2004] and therefore it is very important to control and maintain the initial crystallinity. The structural changes and the crystallinity loss can't be avoided during fine grinding process as it happens concurrently with the size reduction process [THILAGAN PALANIANDY 2008].
The present invention differs from Dobbs and Hsu et al. as they don't take in to consideration the fact that high intensity milling induces structural changes (e.g. crystallinity loss) concurrently with the size reduction process.
The present inventors have also found that even with excessive processing time of such conventional techniques for producing colloidal suspensions, the particle size may not change further. It has further been found that even at conditions and for formulations, where it is possible to obtain a clear colloidal suspension, a significant loss of crystallinity may result, thereby hampering the functionality of the particles in suspension.
Thus, an objective of the present invention is to mitigate the problems of obtaining a more stable and optically clear colloidal suspension of a nano-crystalline particles in a solvent than in the prior art.
It is further an objective to provide a method, which allows for production of colloidal suspensions of nano-crystalline powders in a solvent, which are faster and/or more efficient and/or result in a smaller particle size in said colloidal suspension and/or maintain a higher crystallinity of said particles in said colloidal suspension and/or have a higher functionality than in the prior art.
One of the objectives of the present invention is to control the crystallinity of the nanoparticles without loosing the stability during the production of nanoparticles suspensions.
Still another objective of the present invention is to provide stable and optically clear colloidal suspension products suitable for making clear coatings with improved functionality.